1. HVSTRLEN -> HSTRLEN. It's unlikely one needs the length of the key,
   not clear how the API would work (by value does not make sense) and
   there will be better names anyway.
2. Default is to return 0 when field is missing.
3. Default is to return 0 when key is missing.
4. The implementation was slower than needed, and produced unnecessary COW.

Related issue #2415.

added a new hvstrlen command

Thanks to @codeslinger (Toby DiPasquale) for identifying the issue.

Related to issue #2409.

1. Remove useless "cs" initialization.
2. Add a "select" var to capture a condition checked multiple times.
3. Avoid duplication of the same if (!copy) conditional.
4. Don't increment dirty if copy is given (no deletion is performed),
   otherwise we propagate MIGRATE when not needed.

Avoid redundant SELECT calls when continuously migrating keys to
the same dbid within a target Redis instance.

rehashing.c: Fix compile error originating from SPOP rewrite

This is safe since bufpos is small, inside the range of the local
client buffer.

Bitops: Stop overallocating storage space on set

Sentinel: Add initial quorum bounds check

Revert some size_t back to off_t
Set reply_bytes needs to 64 bits everywhere
Revert bufpos to int since it's a max of 16k into buf[]

Improve type correctness

Sentinel: Improve INFO command behavior

Improve consistency of INFO MEMORY

Fix quicklist Pop()

Fixes issue #2392.

the hvstrlen command returns the length of a hash field value

It's hard to pick a good approach here. A few arguments:

1) There are many exposed instances on the internet.
2) Changing the default when "bind" is not given is very dangerous,
   after an upgrade the server changes a fundamental behavior.
3) Usually Redis, when used in a proper way, will be protected *and*
   accessed often from other computers, so this new default is likely
   not what most people want.
4) However if users end with this default, they are using the example
   redis.conf: likely they are reading what is inside, and they'll see
   the warning.

Now the tests actually compare return values instead of just
verifying _something_ got returned.

Closes #2398

Readme fix

Apparently no refernece-style links supported in Github markdown.

Less grays: more readable palette since usually we have a non linear
distribution of percentages and very near gray tones are hard to take
apart. Final part of the palette is gradient from yellow to red. The red
part is hardly reached because of usual distribution of latencies, but
shows up mainly when latencies are very high because of the logarithmic
scale, this is coherent to what people expect: red = bad.